Camera calibration techniques are known in the fields of robotics and machine vision. According to one existing calibration technique for stereo camera pairs, for example, the determination of a cameras position is based upon knowledge of the absolute position of targets in three space. This procedure enables subsequent measurement through triangulation using the views from the two cameras.
Existing approaches to work-cell type calibration either mandate multiple, stereoscopic cameras or factory calibrated components. For example, U.S. Pat. No. 4,645,348, teaches a laser and a camera which are factory-calibrated or `rectified` with respect to one another and contained within an integrated enclosure. However, although the laser and camera are calibrated with respect to one another, they are not calibrated with respect to a robot. Rather, the inventors in this case expect to supply the calibrated laser/camera combination in component form for user calibration within different application environments.
U.S. Pat. No. 4,639,878 patent uses multiple cameras, but they are independently calibrated with respect to a fixture. Given this fixture, traditional methods are used to calibrate cameras and robot end effectors through touch. In this example there is no direct calibration between the cameras and the robot.
U.S. Pat. No. 5,329,469 patent discloses a position correction arrangement, wherein cameras mounted on the end of the robot arm are moved to different locations, at which point cameras are used for viewing purposes only; that is, differences in spatial position are noted but not used to compute update information.
In U.S. Pat. No. 4,753,569, a robot holds a card exhibiting a dot pattern, which a camera views at a skewed angle in order to ascertain depth information and not absolute position. In this system, not only is information required as to the relationship among the dots, but the relationship between the dots, the card, and the robot arm, must also be determined and input in advance.
Other references require very precise alignment prior to, or during, use, thereby limiting their applicability. U.S. Pat. No. 5,513,276, for example, requires the laser and the angle of view of the camera to be parallel. U.S. Pat. No. 4,979,815 discloses the use of a laser and a camera to obtain height information. Specifically, objects under inspection are scanned to gather depth profiles, requiring the laser to be very accurately maintained normal to the surface of the observation platform. Accordingly, a gage block is inserted in the field of view which is scanned, enabling images to be calibrated relative to the gage block.
Other methods have used fixed cameras and lasers and factory calibration of the combination to provide three-dimensional measurement with respect to the set-up. This limits the available laser-camera configurations to those supplied by the factory, and provides no means for rectification of the data into a robot's coordinate frame.